Electron Transport Chain(Respiratory Chain)

Dr. Usman Ghani1 Lecture

Respiratory Block

Electron Transport Chain (ETC)

• A system of electron transport that uses respiratory O2 to finally produce ATP (energy)

• Located in the inner mitochondrial membrane• Final common pathway of metabolism• Electrons from food metabolism are

transported to O2

• Uses maximum amount of body’s oxygen

Metabolic breakdown ofenergy-yielding molecules

Energy-rich reduced

coenzymes

Electrons(e-) lose their free energy

Excess energy generates heat

An electron micrograph of an animal mitochondrion

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Cutaway diagram of a mitochondrion

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Mitochondrion

Cristae increase the surface area

Components of ETC

• All members/components are located in the inner mitochondrial membrane (IMM)

• IMM contains 5 complexes:– Complex I, II, III, IV (part of ETC)– Complex V (ATP synthase: catalyzes ATP synthesis)– Mobile electron carriers

• CoQ• Cytochrome c

Organization of ETC

• Each complex accepts or donates electrons to mobile carriers

• Carriers accept electrons from donors and then donate to the next carrier in chain

• Electrons finally combine with oxygen and protons to form water

• Oxygen is required as a final acceptor (respiratory chain)

Electron Transport Chain

Complex I – NADH Dehydrogenase

• This complex collects the pair of electrons from NADH and passes them to CoQ

Electron Transport Chain

Complex II – Succinate dehydrogenase

• It is also a part of the TCA cycle• Transfers electrons to CoQ

Electron Transport Chain

Coenzyme Q (CoQ)

• Also called ubiquinone (ubiquitous in biological systems)

• A non-protein member of the ETC• Lipid soluble and mobile

Cytochromes

• Each cytochrome is a protein that contains– Heme group (porphyrin ring + iron in Fe3+ state)

• When cytochromes accept electron– Fe3+ (ferric) is converted to Fe2+ (ferrous)– Fe2+ is reoxidized to Fe3+ when it donates electrons

to the next carrier

Complex III and IV

• Complex III: Cytochrome bc1• Complex IV: Cytochrome a + a3

Electrons flow from:• CoQ Complex III Cyt. c Complex IV

Electron Transport Chain

Site-specific inhibitors of ETC

ETC is coupled to proton transport for ATP synthesis

• The energy of electron transfer is used to drive the protons out of the matrix

• It is done by complexes I, III and IV (proton pumps)

• This creates a proton gradient across the IMM to synthesize ATP

Coupling of electron transport (green arrow)and ATP synthesis

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ATP synthase

• ATP synthase (Complex V) synthesizes ATP• Consists of two domains:

F0 – membrane spanning domain

F1 – extramembranous domain

Transport of protons

Energetics of ATP synthesis

• The energy required for phosphorylation of ADP to ATP = 7.3kcal/mol

• Energy produced from the transport of a pair of electrons from NADH to O2 = 52.58 kcal

• No. of ATP molecules produced is 3 (NADH to O2)

• Excess energy is used for other reactions or released as heat

P:O ratio

• ATP made per O atom reduced–For NADH

• P:O = 3:1–For FADH2

• P:O = 2:1

Inhibitors of ATP synthesis

• Oligomycin:– Binds to F0 domain of ATP synthase and closes the

H+ channel

• Uncoupling proteins (UCPs):– Create proton leaks (allow protons to reenter the

matrix without ATP synthesis)– Energy is released as heat (nonshivering

thermogenesis)